The copying machine shown in FIG. 8 is known as a conventional electrophotographic machine wherein upper and lower heat rollers 914a and 914b are provided on a fixing means 914. The copying machine permits both black-and-white copying and full color copying. The copying machine is arranged so as to scan a document placed on a document platen 903 using a lamp 902 of an optical system 901. Further, a photoreceptor 905 is exposed using a light reflected from the document through a filter 904, thereby forming an electrostatic latent image on the surface of the photoreceptor 905. The electrostatic latent image is developed using a color toner in developers 906 through 908, or using a black toner in a developer 909 so as to form a toner image. The toner image is transferred on a sheet supplied from one of sheet cassettes 911 through 913 through an intermediate transfer belt 910. The upper and lower heat rollers 914a and 914b of the fixing means 914 are in tight contact with one another. The toner of the toner image is melted in a fixing portion provided between the heat rollers 914a and 914b, whereby the toner image is permanently affixed onto the sheet.
On the other hand, the copying machines disclosed in U.S. Pat. No. 3,965,331 and Japanese Laid-Open patent publication 98036/1976 (Tokukaisho 51-98036) have the following arrangement: A fixing means is provided with upper, intermediate, and lower heat rollers. An upper fixing portion is formed between the upper and intermediate heat rollers, and a lower fixing portion is formed between the intermediate and lower heat rollers. A transport belt for conveying the sheet either to the upper fixing portion or to the lower fixing portion can be moved up and down according to the fixing portion to be used. In the above arrangement, by selecting an appropriate material for the upper, intermediate, and lower heat rollers, the copying machines can be made applicable to recording material made of various material such as paper, polycarbonate, polyethylenetelephthalate, etc.
However, in the above-type of fixing means provided with the three heat rollers, when both of the upper and lower fixing portions are made applicable to both full color copying and black-and-white copying in order to enable both full color copying and black-and-white copying, the following problems will arise as in the case of the aforementioned type of fixing means provided with two heat rollers.
Because a control is required in each fixing portion according to a selected copying mode, i.e., black-and-white copying or full color copying, the control system becomes complicated, or an appropriate fixing process is difficult to be obtained.
Because the heat roller for full color copying is arranged so as to make a three-layered color toner adhere onto the sheet without adhering onto the surface of the heat roller, i.e., without offsetting the toner image, a smooth surface is required for the heat roller. This results in a higher manufacturing cost and a lower durability compared with the heat roller for black-and-white copying. Therefore, in the case where the heat roller for full color copying is adopted for both upper and intermediate heat rollers in order to make the upper and lower fixing portions applicable to both full color copying and black-and-white copying, a frequent exchange is required for both of the heat rollers. Moreover, a high cost is required for processing the black-and-white copying.
Furthermore, compared with the black toner used in the monochrome copying, the softening temperature and the melting viscosity of the color toner used in the full color copying are required to be set significantly lower in order to fully mix it for the three-layered color toner. Therefore, the offset is likely to be generated, and in order to surely prevent this, a large amount of silicone oil is required for both of the upper and intermediate heat rollers when a fixing process for full color copying is carried out in both of the upper and lower fixing portions. For this reason, in both cases where the fixing process for black-and-white copying is carried out in the upper fixing portion and in the lower fixing portion, the silicone oil used in the fixing process for the full color copying adheres onto both surfaces or one surface of the sheet. Furthermore, when copying on both sides of the sheet, the silicone oil adhering onto the sheet will adhere onto a transport roller in a retransport path. If this happens, the transporting ability is lowered, and the sheet is likely to get stuck in the apparatus. Moreover, if the silicone oil adheres onto the photoreceptor, etc., or gets inserted in the developer vessel, the image quality will be lowered.
In the copying machines designed for both black-and-white copying and full color copying, in order to prevent the above problems, the fixing means provided with the upper, intermediate, and lower heat rollers is arranged such that either one of the upper and lower fixing portions is associated with the black-and-white copying, and the other fixing portion is associated with the full color copying as disclosed in Japanese Laid-open Patent application 191979/1990 (Tokukaihei 2-191979) and U.S. Pat. No. 4,928,148.
However, with the above-type of copying machine, in the case where a reverse and retransport unit is provided for reversing the recording material discharged from the fixing means and retransporting it to the main body in order to enable the image to be formed on both sides of the copying material in the black-and-white copying process, the transport path for the copying material discharged from the fixing means becomes highly complicated.
More concretely, with the above arrangement, two discharge transport paths for guiding the recording material out of the device, one being associated with the full color copying and the other being associated with the black-and-white copying, and a retransport path for guiding the copying material to the reverse and retransport unit in the both sided copying mode are provided on the recording material discharge side of the fixing means. In this case, since these paths cross, the structure of the paths is complicated, and the apparatus becomes larger in size. Thus, a high manufacturing cost is required. Moreover, the copying material is likely to get stuck in the apparatus, and the clearing of the copy material when this happens is also difficult.
Depending on the use of the copied image, such copying may be demanded wherein a black-and-white image is copied on one side of the recording material, and a full color image is copied on the other side of the copying material. Therefore, the development of a device which enables the above copying without presenting the above problems is desired.
Furthermore, in a successive image forming process wherein single sided black-and-white image forming processes and the single sided full color image forming processes are combined (mixed copying process) with respect to a plurality of documents including monochrome documents and color documents, the following problems arise. Each time switching from the full color copying to black-and-white copying, or from the black-and-white copying to the full color copying, the transport belt is required to be moved. Moreover, because each of the heat rollers is switched off when it is not used, readjustment of the temperature is required. Therefore, a long time is required for the mixed copying process.